A number of systems and programs are offered on the market for the design, the engineering and the manufacturing of objects. CAD is an acronym for Computer-Aided Design, e.g. it relates to software solutions for designing an object. CAE is an acronym for Computer-Aided Engineering, e.g. it relates to software solutions for simulating the physical behavior of a future product. CAM is an acronym for Computer-Aided Manufacturing, e.g. it relates to software solutions for defining manufacturing processes and operations. In such systems, the graphical user interface (GUI) plays an important role as regards the efficiency of the technique. These techniques may be embedded within Product Lifecycle Management (PLM) systems. PLM refers to a business strategy that helps companies to share product data, apply common processes, and leverage corporate knowledge for the development of products from conception to the end of their life, across the concept of extended enterprise.
The PLM solutions provided by Dassault Systemes (under the trademarks CATIA, ENOVIA and DELMIA) provide an Engineering Hub, which organizes product engineering knowledge, a Manufacturing Hub, which manages manufacturing engineering knowledge, and an Enterprise Hub which enables enterprise integrations and connections into both the Engineering and Manufacturing Hubs. All together the system delivers an open object model linking products, processes, resources to enable dynamic, knowledge-based product creation and decision support that drives optimized product definition, manufacturing preparation, production and service.
Objects designed by the above systems are usually modeled by a 3D surface. Existing solutions for interactive design of surfaces include principally two technologies.
On the one hand, many solutions are based on the very popular non uniform rational B-spline surfaces (NURBS), such as the ones presented in the following papers:    Bézier, P. (1987). Courbes et surfaces. Paris, Hermès;    Casteljau, P. D. (1985). Formes à Pôles. Paris, Hermès;    Farin, G. (1993), Curves and surfaces for computer aided geometric design: a practical guide. San Diego, USA, Academic Press Professional, Inc; or    Piegl, L. L., Tiller W. (1996), The NURBS book. Springer.
Mainly, a NURBS surface is defined by a rectangular grid of control points together with other parameters to manage smoothness. Once the smoothness parameter is set, in order to change the shape of the surface, the user moves control points, which in turn changes the shape of the surface. In other words, the control points are the intermediate tool to edit the surface. The resulting surface is a rectangular patchwork of polynomial or rational patches. By nature, a patch surface features a rectangular topology, being bounded by four edges. This model is very popular because the relationship between control points and the resulting surface is very natural and intuitive. Furthermore, local influence of control points allows the user to design local details on the surface.
On the other hand, many solutions are based on subdivision surfaces such as the ones presented in the following papers:    E. Catmull and J. Clark, Recursively generated B-spline surfaces on arbitrary topological meshes. Computer-Aided Design, 10:350-355, July 1978;    J. Stam, Exact Evaluation Of Catmull-Clark Subdivision Surfaces At Arbitrary Parameter Values. Proceedings of SIGGRAPH 1998, pages 395-404, July 1998;    Jordan Smith and Carlo Sequin, Eigen Structure of Stationary Subdivision Schemes and Differential Geometry of Surfaces;     Jörg Peters, Patching Catmull-Clark Meshes, University of Florida; and    Jörg Peters, Modifications of PCCM, University of Florida.
A subdivision surface is defined by a polygonal mesh, named the “base mesh” in the following, which controls the shape of the surface. Nevertheless, is differs from the NURBS technology as explained in the following. Firstly, the topology of the base mesh is not restricted. It can be closed or open and can feature as many holes as needed. Secondly, the resulting subdivision surface from this base mesh is a theoretical surface that is the limit surface of an infinite and convergent subdivision process starting with the base mesh. Despite the actual subdivision surface is out of reach from the theoretical point of view, it can be approximated with a very good precision by a set of adjacent NURBS surface patches, as detailed in documents U.S. Pat. No. 7,595,799, U.S. Pat. No. 7,400,323 or EP1750229. Here again, local reshaping is allowed, but by refining the base mesh and by moving control points of the base mesh.
As opposed to mechanical or functional surfaces, styling surfaces are based on a detailed design (i.e. “styling design”), resulting in a surface with many and precise geometric features. One goal of styling design is to create a so-called “character line” on the surface. A character line is a sharp and long fold that runs on the surface. When this feature ends in the interior of the surface, a so-called “transition zone” is where the sharp fold gets smoother and smoother in order to vanish within the neighboring surface. Both NURBS and subdivision surfaces can design a character line by adjusting control points.
The subdivision surface technology is preferred for styling design because of its unrestricted topology (as opposed to the rectangular topology of a NURBS surface). A problem is that the resulting shape of the character line and its transition zone is not satisfactory when using a subdivision surface. Tests performed on industrial styling surfaces show that a skilled designer is not able to adjust the base mesh in order to get the expected character line and transition zone. Consequently, and using prior art technology, the existing solution is to (1) create the approximate NURBS surface approximating the subdivision surface, (2) remove manually from this NURBS surface the (unsatisfactory) transition zone, (3) design a local NURBS surface featuring the correct shape and (4) join this local surface to the surface from step (2).
Clearly, this methodology provides a correct result. The drawback is that if the initial subdivision surface is modified afterward, the transition zone must be created again on the modified surface. The designer is forced to run again steps (2) to (4) by hand, which is time consuming. This is because steps (2) to (4) are not captured through a history by the CAD system and thus cannot be replayed automatically when the input surface is changed.
Thus, the solutions listed above lack efficiency, notably from a user utilization point of view. Within this context, there is still a need for an improved solution for designing a 3D modeled object modeled by a subdivision surface.